Let me first preface this by saying that I am not an expert on creating calkits or editing calkits. There are others on the forum who are much more knowledgeable on this particular topic and hopefully they will chime in. But here is what I know:

GCA calibration is what we call a guided cal and as a guided cal, the way calkits are selected is by first selecting the right connector for each port in the cal and choosing the corresponding calkit the matches the connector for that port. So if you are creating your own calkit then you have to make sure that you associate that with the right connector. If I am not mistaken most GPPO connectors are gendered (they have male and female types), so your cal kit definition has to cover both genders of the connector.

Secondly when you create a new cal kit or edit an existing one, any standards you add are by default added to the SOLT definition even if you have selected the TRL class assignment. So if you want to be sure that the calibration performed by GCA when you select your kit is TRL, then you have to edit the SOLT class assignment in your calkit and delete any standards that are added there. If the only standards defined in your kit are TRL, then any guided cal using that kit will be forced to perform a TRL cal. The preference in the guided calibration engine is to do SOLT cals, so if there are any SOLT standards present in the kit, the calibration method will default to SOLT (or some variant of SOLT). The best way to ensure a TRL cal, is to not have any SOLT standards in the first place. However, if your kit is defined properly with both TRL and SOLT standards, in GCA as well as other guided cal wizards, there is always a check box that is usually labeled something like "Modify Cal: Change cal method, standards,...". if you check this box, the subsequent pages in the cal wizard will allow you to select the TRL standards for the calibration.

Let us know if you are still having problems performing your TRL GCA calibration.

To add on to Dara's response, be careful that the thru type is set to Defined Thru, Flush Thru, or Adapter Removal (with the adapter removal thru types set to TRL). Most GPPO custom kits I've seen use males on both cables and females on the standards so I would recommend using Defined Thru. I set a test up with the 85052C SOLT/TRL 3.5mm cal kit and it defaulted to TRL. As Dara mentioned you will need to get the connector selection right and make sure the kit is setup properly. You could post or PM your cal kit file (.ckt) and I'll take a look to make sure the definitions are at least defined correctly (assuming the values are correct).

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Check the frequency range assigned to the TRL Reflect standard (Short), it is defined as only supporting 512 MHz to 4 GHz, where the other standards are defined from 4 GHz to 16 GHz. Once I changed this, it defaulted to TRL and the options for Enhanced Response also appeared.

I should have mentioned that ..I fixed that on the PNA...so for the standard S-PAR Measurement Class I see TRL and enhance response...but for theGCA Measurement Class, I only see standard cal kits, since I cannot change the connector types

Can you try it with the GCA Measurement Class, and tell me the steps you used?

Since the connectors in the cal kit should match what is on the DUT and all of your standards have APC 3.5 male defined as the connector type for all standards, it would be APC 3.5 male for the DUT connector selection in the calibration process.

The reference plane of GPPO connectors are ill defined and therefore not very repeatable from device to device. Adapter substituion method may be more repeatable and traceable. I am not aware of any GPPO cal kit that performed adequately. See paper "Evaluating the Sutability of OSP Connectors for Calibratio Kit Use" by Paul R. Pino, RF Globalnet, Nov. 2, 2006.